Tandem Lift Load Share and Outrigger Ground Bearing
An 18,000-lb precast wall panel is to be set with two 50-ton rough-terrain cranes in a tandem lift. The pick points are 24 ft apart. The panel's center of gravity is 9 ft from the Crane A pick point and 15 ft from the Crane B pick point. Crane A is set at a 30-ft radius, where its load chart allows 12,000 lb; its rigging weighs 1,200 lb. Crane B is at a 28-ft radius, where its chart allows 13,500 lb; its rigging weighs 1,000 lb. The lift plan shows no calculation of load share and no outrigger ground-bearing check. The ground under Crane A's outrigger is compacted fill with an allowable bearing pressure of 4,000 psf; the outrigger float is 2 ft by 2 ft and the reaction is estimated at 34,000 lb. Wind is 18 mph, and the panel has 240 sq ft of sail area. The crew wants to proceed because each crane's chart capacity is greater than half the load.
1. Calculate each crane's load share and net capacity, and determine whether the lift is acceptable.
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Load share is inversely proportional to distance from the CG. Crane A share = 18,000 x 15/24 = 11,250 lb; Crane B share = 18,000 x 9/24 = 6,750 lb. Crane A net capacity = 12,000 - 1,200 = 10,800 lb. The 11,250-lb share exceeds net capacity by 450 lb (about 4.2%), so Crane A is overloaded. Crane B net capacity = 13,500 - 1,000 = 12,500 lb, which covers 6,750 lb. Ground bearing also fails: 34,000 lb / (2 ft x 2 ft) = 8,500 psf, more than twice the 4,000-psf allowable. The 'chart capacity greater than half the load' check is invalid for a tandem lift with an offset CG. The lift should not proceed.
2. What are viable corrective actions, and what must be re-verified before the lift?
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Options include using a larger crane for A, moving A to a smaller radius with a verified chart, changing pick points to move the CG, or using an equalizing beam or engineered lift arrangement. Any change must be recalculated: actual load share, all rigging deductions, boom length, outrigger configuration, and ground bearing. If A is moved to 27 ft and capacity rises to 13,000 lb, net becomes 11,800 lb, only 550 lb above the 11,250-lb share; that margin is thin and still requires ground improvement. A 4-ft by 4-ft mat would reduce 34,000 lb to about 2,125 psf, but the mat and soil must be evaluated by a qualified person. Wind and sail area must be checked against the manufacturer's limits, and a written two-crane lift plan with lift director, communications, and tag lines is required.
3. The site supervisor says the 4,000 psf is conservative and the outrigger will 'settle in.' How do you respond and document this?
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Settlement is not a benign adjustment. If an outrigger sinks, the crane tilts, the load share between cranes changes, and side loads develop in the boom and rigging. The 4,000-psf value must be treated as a limit unless a geotechnical or qualified person verifies higher capacity; a verbal assurance is not documentation. Require outrigger mats sized to distribute the reaction, a level and stable supporting surface, and verification of actual bearing pressure. Record the rejection, the calculated 8,500-psf demand, and the required corrective actions in the lift plan. Do not allow the lift until the ground support and load-share calculations are signed off.